Digitizing tablet system including a tablet having a grid structure made of two orthogonal sets of parallel uniformly sized and spaced U shaped loops of conductive material

ABSTRACT

A digitizing tablet system having a stylus which is inductively coupled to a grid is disclosed. The grid includes a first plurality of narrow uniformly sized and spaced U shaped loops of conductive material disposed to define a first coordinate axis of the grid and a second plurality of narrow uniformly sized and spaced U shaped loops of conductive material disposed to define a second coordinate axis of the grid. One end point of each loop in each plurality of loops is connected to a common conductor which serves as one output terminal for that plurality of loops. The other end point of each loop in the same plurality of loops is connected to a multiplexor whose output is connected to a conductor which serves as the other output terminal for that plurality of loops. Voltage signals induced in the loops when the stylus is positioned above the grid and a coil in the stylus is energized are processed to generate data signals corresponding to the position of the stylus relative to the grid structure. Since the loops are uniform in size and shape, nonlinearities in the relationship between the induced voltage and the computed stylus position from one loop to the next are eliminated. In addition, because the loops are narrow in size the amount of stray electromagnetic interference picked up by the loops from external sources is minimized.

This is a continuation of co-pending application Ser. No. 592,879, filedon Mar. 23, 1984, now U.S. Pat. No. 4,582,955.

BACKGROUND OF THE INVENTION

The present invention relates generally to digitizing tablet systems andmore particularly to digitizing tablet systems which include aconductive grid structure and a stylus structure which is inductivelycoupled to the conductive grid structure.

Digitizing tablet systems are well known in the art and are used in avariety of applications. These systems generally include a tablet, astylus and some instrumentality for producing some form of interactionbetween the stylus and the tablet from which is derived data signalsrepresenting the position of the stylus on the tablet.

In a number of these systems the tablet includes a housing having a twodimensional work support surface and a grid of conductive elements andthe stylus contains an electric coil. The coil in the stylus isinducively coupled to the grid in the tablet by energizing either thecoil or the grid with an AC voltage signal. The voltage signal inducedin the other component is then measured. The measured signals are thenused to determine the position of the stylus relative to the grid.

The grid in inductive coupling type digitizing tablet systems very oftencomprises a first set of parallel equally spaced straight wires and asecond set of parallel equally spaced straight wires which are at rightangles to the first set of wires. One end of each wire in each set isconnected to a common conductor having one end which serves as oneterminal for the set. The other end of each wire in the set is connectedto a multiplexor or other similar device having an end which isconnected to a conductor which serves as the other terminal for the set.The multiplexor allows the individual wires in the set to be enabledsequentially. The two terminals for each set are coupled to theelectronics portion of the system where signal processing is performed.By energizing the coil in the stylus, voltages will be induced in theindividual wires or, alternatively, by energizing the individual wires,voltages will be induced in the coil in the stylus. The induced voltagesignals are processed to produce data signals representing the positionof the stylus relative to the grid.

In the past, it has been considered that the size of the induced voltagesignal is directly related to the proximity of the coil in the stylus tothe particular wire in the grid from which the induced voltage signal isderived. The problem with this line of reasoning is that it is notentirely correct. More specifically, the induced voltage generated in orfrom a wire is not related exclusively to the proximity of the coil inthe stylus to the particular wire but rather to the overall loop definedmainly by the particular wire in the grid wire but also including thecommon conductor, the multiplexor and the connections from the commonconductor and the multiplexor to the associated electronics. Since theindividual wires are straight, equally spaced from one another, andconnected at opposite ends to common elements, the size or area of theoverall loop related to each individual wire will be different eventhough the distance between wires is the same. Because the loops aredifferently sized, the induced voltages related to each wire will bedifferent. For example, assume a voltage signal of 1 volt is applied tothe coil of the stylus and that the stylus is positioned 0.1 inches froma wire in the grid which forms part of an overall loop having an area of12 square inches and that the induced voltage detected by that wire is0.1 volts. If the stylus is positioned 0.1 inches from another wire inthe grid which forms an overall loop having an area of 80 square inches,the induced voltage detected by that wire will not be 0.1 volts but willbe somewhat smaller, such as 0.04 volts.

In addition, as the size of the overall loop increases, the amount ofstray fringe fields picked up by the loop also increases.

As can be appreciated, both of the above phenomina will producenonlinearities or errors in the relationship between the induced voltagesignals and the computed stylus position which in many applications areundesirable and/or unacceptable.

In U.S. Pat. No. 4,243,843 to R. T. Rocheleau there is disclosed anapparatus for determining the distance of a point on a surface from anaxis on the surface comprising a cursor having a magnetisable elementadapted to be energized by an alternating voltage, a grid ofsubstantially parallel spaced conductors, the cursor being movablethereover and inductively coupled thereto, means for sequentiallyenabling only one of said conductors to conduct a current induced by thealternating voltage at any one time, means for comparing percentages ofthe amplitudes of currents induced in successively enabled conductors,means for sensing a change in the phase of the induced current, andmeans responsive to the comparing means and the sensing means forcounting the number of conductors which have been enabled at the time aphase change is sensed.

In U.S. Pat. No. 3,873,770 to J. T. Ioannou there is disclosed a systemfor digitizing graphic data from a worksheet by tracing out or pointingto curves and points on the worksheet with a stylus. A tablet having asurface to receive the worksheet includes a conductor grid defining twoperpendicular axes of measurement. The conductors are sequentiallyexcited and a coil in the stylus picks up an impluse having an envelopewhich shows positive and negative peaks spaced by a distance 2 h, whereh is coil height above the grid plane measured along the stylus axis.The conductor grid planes are physically displaced from the tabletsurface by some small but finite distance thus giving rise to anapparent position error if the stylus is tilted during use; i.e., theindicated position will be the projected intersection of the stylus axiswith the grid plane, not the surface. Means are provided for detectingstylus tilt and variations in h and to compensate for position errors.

In U.S. Pat. No. 3,647,963 to K. N. Bailey there is described a devicefor determining position coordinates of points on a surface whichincludes a conducting grid structure having at least two grid elementsto be placed over or under a surface and a cursor structure having acircular conducting loop element to be moved across the surface of thegrid structure. An alternating electric signal is supplied to either thecursor conducting loop or to each of the conducting grid elements. Thissignal induces a signal in each element of the unexcited conductingstructure. Position coordinates are determined by apparatus whichmeasures the induced signal or signals and records the signal changeproduced when the cursor is moved across the grid surface. Severalembodiments of measuring devices which determine the distance betweenarbitary points on a surface such as a map, graph or photograph areillustrated. Automatic plotting embodiments are also shown and describedin which the plotting motion is determined by comparing signalsrepresenting the measured loop position on the grid with a preselectedset of command signals.

In U.S. Pat. No. 3,705,956 to M. L. Dertouzos there is described agraphic data tablet comprising a tablet having first and second oppositefaces; at least first and second conductor means mounted to said firstface of said tablet, said first conductor means disposed in a first pathcorresponding to a first bit in the Gray Code, said first path occupyinga first area of said first face, said second conductor means disposed ina second path corresponding to a second bit in the Gray Code, saidsecond path occupying a second area of said first face, said first andsecond areas being separate and distinct, said first and secondconductor means being coplanar; means for sequentially passing a signalthrough said first and second conductor means; and sensing meanselectromagnetically communicating with said first and second conductormeans for sensing said signal sequentially passed through said first andsecond means.

Other known patents of interest include U.S. Pat. No. 3,974,332 to Abeet al; U.S. Pat. No. 4,368,351 to S. E. Zimmer; U.S. Pat. No. 3,975,592to P. C. Carvey; U.S. Pat. No. 3,999,012 to H. Dym; U.S. Pat. No.3,700,809 to D. J. Nadon; U.S. Pat. No. 3,732,369 to W. L. Cotter; U.S.Pat. No. 3,735,044 to R. M. Centner; U.S. Pat. No. 3,399,401 to T. O.Ellis et al; U.S. Pat. No. 4,213,005 to E. A. Cameron; and U.S. Pat. No.4,289,927 to J. L. Rodgers.

It is an object of this invention to provide a new and improveddigitizing tablet system.

It is another object of this invention to provide a new and improvedinductive coupling type digitizing tablet system.

It is still another object of this invention to provide a new andimproved grid structure for an inductive coupling type digitizing tabletsystem.

It is yet still another object of this invention to provide a gridstructure for an inductive coupling type digitizing tablet system whichdoes not produce nonlinear relationship between induced voltage signalsand the computed stylus position.

It is a further object of this invention to provide a grid structure foran inductive coupling type digitizing tablet system in whichinterference from stray magnetic fields is minimized.

It is another object of this invention to provide a grid structure foran inductively coupled digitizing tablet system in which the overallloops containing the conductive elements are substantially equal in sizeand shape.

It is still another object of this invention to provide a technique forinterpolating induced voltage signals derived from a grid of uniformlysized and spaced U shaped loops of conductive material to determineposition.

SUMMARY OF THE INVENTION

A digitizing tablet system constructed according to the teachings of thepresent invention includes a stylus structure having a magnetizableelement, a housing having a two dimensional work surface, a gridstructure in the housing, said grid structure including a firstplurality of uniformly sized and spaced U shaped loops of conductivematerial disposed in a plane parallel to said work surface and defininga first coordinate axis and a second plurality of uniformly sized andspaced U shaped loops of conductive material disposed in a planeparallel to said work surface and defining a second coordinate axis,means for energizing one of said grid structure and said stylusstructure with an alternating voltage and producing by electromagneticinduction voltage signals in the other one of said structure related tothe position of said stylus structure relative to said grid structure,and processing means for processing said induced voltage signals toproduce data signals corresponding to the position of the stylusstructure relative to the grid structure.

The foregoing and other objects and advantages will appear from thedescription to follow. In the description, reference is made to theaccompanying drawing which forms a part thereof, and in which is shownby way of illustration, a specific embodiment for practicing theinvention. This embodiment will be described in sufficient detail toenable those skilled in the art to practice the invention, and it is tobe understood that other embodiments may be utilized and that structuralchanges may be made without departing from the scope of the invention.The following detailed description is, therefore, not to be taken in alimiting sense, and the scope of the present invention is best definedby the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings wherein like reference numerals represent like parts:

FIG. 1 is a schematic view of a typical prior art straight wire typegrid configuration used in an inductive coupling type digitizing tabletsystem;

FIG. 2 is a schematic view of the overall loop which includes thestraight wire at one end of the grid configuration shown in FIG. 1;

FIG. 3 is a schematic view of the overall loop which includes thestraight wire at the other end of the grid configuration shown in FIG.1;

FIG. 4 is a simplified view partly broken away in parts of a digitizingtablet system constructed according to the teachings of the presentinvention;

FIG. 5 is a schematic view of the grid structure and associatedelectronics in the digitizing tablet system shown in FIG. 4;

FIG. 6 is a schematic view of the overall loop containing the loopelement at one end of one of the sets of loop elements in the gridstructure shown in FIG. 5;

FIG. 7 is a schematic view of the overall loop containing the loopelement at the other end of the same set of loop elements in the gridstructure shown in FIG. 5;

FIG. 8 is a schematic view of another embodiment of the grid structureof the invention; and

FIGS. 9 and 10 are graphs useful in understanding how readings from aset of loop shaped conductive elements are interpolated to determine theexact position of the stylus relative to the grid.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

The present invention is directed to a digitizing tablet system whichincludes a conductive grid structure and a stylus structure having amagnetizable element which provides inductive coupling to the conductivegrid structure when either one of the conductive grid structure or themagnetizable element are energized and wherein the conductive gridstructure is configured to avoid nonlinearities in the relationshipbetween the induced voltage signal and the computed stylus positionrelative to the grid structure and is configured to minimize noise andinterference in the induced voltage signals caused by extraneoussources. The present invention accomplishes this by providing a gridstructure which is configured to define a pair of orthogonal sets ofnarrow parallel uniformly sized and spaced U shaped loops of conductivematerial. The loops are connected to the electronics position of thesystem in an arrangement whereby the overall area of the loop containinga loop element is essentially equal to the area of the loop elementitself. In use, induced voltage signals derived from the individualconductive loop elements are processed to produce data signalscorresponding to the position of the stylus relative to the grid of loopelements. The processing includes an interpolation technique based on abarabolic curve assumption to determine the coordinates of the positionwherein the induced voltage signal would be at a maximum.

Referring now to the drawings, there is illustrated in FIG. 1 aschematic diagram of a portion of a typical prior art digitizing tabletsystem of the type wherein a coil structure is inductively coupled to agrid structure, the system being identified generally by referencenumeral 11. For simplicity, the wires defining only one of thecoordinate axes are shown.

System 11 includes a grid 13 having a plurality of parallel uniformlyspaced straight wires 15-1 through 15-10. Each wire 15 is coupled at oneend with a respective switch 17-1 through 17-10 in a multiplexor 19. Theend of each wire 15 opposite the end connected to the respective switch17 is connected to a common conductor 21 which serves as one output lineor terminal or grid 13. Multiplexor 19 is connected to another conductor23 which serves as a second output line or terminal of grid 13. Anamplifier 25 has two inputs 27 and 29 which are respectively coupled toconductors 21 and 23. Disposed above grid 13 is stylus (not shown)having a coil 31.

In the past, it has been considered that the voltage induced in eachwire 15 is dependent exclusively on the proximity of the coil 31 to theparticular wire 15. However, as noted above, the induced voltage signalis not dependent exclusively on the wire alone but rather the overallloop which contains the wire.

For convenience, the overall loop containing wire 15-1 is shownseparately in FIG. 2 and the overall loop containing wire 15-10 is shownseparately in FIG. 3. The loop containing wire 15-1 includes conductor21, wire 15-1, switch 17-1 common line 17-11 in multiplexor 19 andconductor 23. On the other hand, the loop containing wire 15-10 includesthe section of conductor 21 from terminal 27 in amplifier 25 to junction28, wire 15-10, switch 17-10, the section of line 17-11 from junction 30to terminal 32 and conductor 23.

As can be seen, the loop containing wire 15-1 is considerably larger inarea than the loop containing wire 15-10. Consequently, the relationshipof the voltage signal induced in the loop containing wire 15-1 when coil31 is at certain distance from wire 15-1 to the stylus position will notbe the same as the relationship of voltage signal induced in the loopcontaining wire 15-10 to the stylus position when coil 31 is at the samedistance from wire 15-10. Furthermore, the loop containing wire 15-1will pick up more stray fringe magnetic fields than the loop containingwire 15-10, because of its larger size.

For illustrative purposes, the invention will hereinafter be describedwith reference to an inductively coupled digitizing tablet system inwhich voltage signals are applied to the coil in the stylus causingsignals to be induced in the grid. However it is to be understood thatthe voltage signals may be applied to the grid rather than the coil inthe stylus, causing signals to be induced in the coil in the stylusrather than the grid.

Referring now to FIG. 4 there is illustrated a digitizing tablet systemconstructed according to the teachings of this invention and identifiedgenerally by reference numeral 41.

System 41 includes a tablet 43 and a stylus 45. Stylus 45 includes acoil 47 which is coupled to the electronics in tablet 43 by a pair ofwires which are shown for simplicity as a single wire 49. Tablet 43includes a generally rectangularly shaped housing having a top flatsurface 51 on which may be placed a worksheet (not shown). The worksheetis maintained in place by any suitable means (also not shown). Disposedinside tablet 43 is a conductive grid structure 53 and the electronics55. Grid structure 53 is spaced below top surface 51 a finite distance,such as around 0.125".

Referring now to FIG. 5 there is illustrated the details of the gridstructure 53 and the associated electronics 55.

Grid structure 53 includes a first plurality of elongated narrowparallel U shaped conductive loops (identified by reference numerals)61-1 through 61-10, the number of loops shown being for illustrativepurposes only. Loops 61-1 through 61-10 are uniformly spaced, equal insize and shape and disposed in a plane parallel to surface 51, definingone coordinate axis, or the "X" axis of measurement. Each loop 61includes a first leg or longitudinal loop section, a second leg orlongitudinal loop section and a first transverse or connecting loopsection. The two longitudinal loop sections are parallel. In loop 61-1the first and second longitudinal loop sections and the transverse loopsections are identified as 61-1A, 61-1B and 61-1C respectively.

Grid structure further includes a second plurality of elongated parallelnarrow U shpaed conductive loops 63-1 through 63-10, the number of loopsshown also being for illustrative purposes only. Loops 63-1 through63-10 are uniformly spaced, equal in size and shape and disposed in aplane parallel to and spaced from loop elements 61-1 through 61-10 atright angles thereto and defining a second coordinate axis, or the "Y"axis of measurement. The loop size and spacing of both pluralities ofloops are preferably the same.

Loops 61 and 63 may either be individual wires of conductive material orlengths of conductive material on a printed circuit board.

One end point E₁ of each loop 61 is connected to a common conductiveelement 65 which serves as one output terminal or line for loops 61 andthe other end point E₂ of each loop 61 is connected to a multiplexor 67having an output line 69 which serves as the other output terminal orline for loops 61. Output line 65 is very close to multiplexor 67 andthe two output lines 65 and 69 are very close to each other. Similarly,loops 63 are connected to a common conductive element 71 and amultiplexor 73 having an output line 75.

The overall loop containing loop 61-1 is shown separately in FIG. 6 andthe overall loop containing loop 61-10 is shown separately in FIG. 7.Since conductive element 65 is disposed physically very close tomultiplexor 67 and output line 69 the area between wire 65 andmultiplexor 67 is very small or negliable. Thus, the overall loop areaof the loop containing loop 61-1 is essentially equal to the area ofloop 61-1 and the overall loop area of the loop containing loop 61-10 isessentially equal to the area of loop 61-10. Similarly, the overall loopareas of each one of the loops containing loop 63-1 through 63-10 isessentially equal to the areas of loops 63-1 through 63-10.

As can be appreciated, since the loop elements in each group areidentical in size and shape, the signal induced in one loop, such asloop 61-1, when coil 47 is a certain distance therefrom will be the sameas the signal induced in another loop such as loop 61-5 when coil 47 isat the same distance therefrom. In addition, since all loops are equalin size, the stray noise or fringe fields picked up by the loops will beabout the same for each loop. Furthermore, since the loops arerelatively narrow the stray noise or fringe fields will be at a minimum.

Coil 47 has a diameter that is slightly less than the width of a loop.

In use, the maximum signal induced in a loop will occur when the coil 41is aligned directly between the two longitudinal wires of a loop.

Conductive elements 65 and 69 are connected to an amplifier 77 whoseoutput is fed into a demodulator and filter combination 79. The outputof demodulator and filter combination 79 is fed into a A/D converter 81.The output of A/D converter 81 is fed into a processor 83. Conductiveelements 71 and 75 are connected to an amplifier 85 whose output is fedinto a demodulator and filter combination 87. The output of demodulatorand filter combination 87 is fed into a A/D converter 89 whose output isalso connected to processor 83.

The output of processor 83 is connected to a computer or displayterminal 83.

Coil 41 is coupled to an AC voltage source 91.

In FIG. 8 there is illustrated another embodiment of grid structureidentified by reference numeral 101. For simplicity, only one of thesets of loop elements are shown. The set includes a plurality of loopelements 103-1 to 103-5 which are identical in size and shape to loopelements 61. The number of loops in the set is for illustrative purposesonly.

Loops 103 are uniformly spaced but are disposed in overlappingrelationship. Because the loop elements are disposed in overlappingrelationship, the number of loop elements and hence the number ofreadings that may be obtained for a given length along the axis ofmeasurement is increased.

In the operation of digitizing tablet system 41, a voltage signal fromAC voltage source 91 is applied to coil 47 in stylus 45 producing fluxpatterns linking coil 47 to grid 53. This will induce voltages in theloop elements of grid 53. The amplitude of the induced voltage signalsin each loop element will be a function of the distance from coil 47 tothe particular loop element. The largest induced voltage signal will begenerated from a loop element when coil 47 is positioned directly overthe loop element.

The voltage signals generated in loop elements 61 are fed sequentiallyinto amplifier 77. The amplified voltage signal outputs from amplifier77 are filtered and demodulated by filter and demodulator unit 79 andthen converted into digital signals by A/D converter 81. The output datasignals from A/D converter 81 are fed into processor 83. The inducedvoltage signals from loop elements 63 are similarly amplified, filtered,demodulated, converted to digital signals and fed into processor 83. Inprocessor 83 the two sets of data signals, one from loop elements 61 andthe other from loop elements 63 are processed to produce data signalscorresponding to the exact position of stylus 45 relative to grid 61.

The processing in processer 83 involves determining the peak of aparabolic curve defined by three points or readings, the first pointcorresponding to the largest induced voltage signal detected and theother two points corresponding to the amplitudes of the induced voltagesignals from the two loop elements on either side of the loop elementproducing the largest induced voltage signal.

The processing in processor 83 may be best understood by referring toFIGS. 9 and 10.

FIG. 9 is a graph of voltage amplitude vs. loop element position forloop elements 61-1, 61-2 and 61-3 when coil 47 of stylus 45 is directlyover the center of coil element 61-2. In the graph, point R₁ representsthe amplitude of the induced voltage signal from coil element 61-1,point S₁ represents the amplitude of the induced voltage signal fromcoil element 61-2 and point T₁ represents the amplitude of the inducedvoltage signal from coil element 61-3. P₆₁₋₁ is the position of loopelement 61-1 along the X-axis, P₆₁₋₂ is the position of loop element61-2 along the X axis and P₆₁₋₃ is the position of loop element 61-3along the X axis. The three points R₁, S₁ and T₁ are joined together bya parabolic shaped curve Z₁. As can be seen, point S₁ is at the peak CP₁of curve Z₁ and the stylus position P_(ST1) is at the center of coil61-2.

FIG. 8 illustrates a graph of the induced voltages from the same threecoil elements when coil 47 is positioned between coils 61-1 and 61-2.The three amplitude points, labelled R₂, S₂ and T₂, are joined togetherby a parabolic curve Z₂. In this case, the peak CP₂ of the paraboliccurve Z₂ is between points R₂ and S₂ and the stylus position P_(ST2) isbetween the position P₆₁₋₁ of loop 61-1 and the position P₆₁₋₂ of loop61-2.

The stylus position corresponding to the peak of the parabolic curve maybe calculated by the formula: ##EQU1## Where: P_(Stylus) =the actualposition of the stylus along the axis of measurement.

P_(s) =The position along the axis of measurement of the loop elementhaving the largest detected induced voltage signal.

L_(d) =the distance between loop elements along the axis of measurement.

V_(s) =the amplitude of the largest detected voltage.

V_(r) =the amplitude of the voltage detected by the loop element on oneside of the loop element having the largest detected voltage.

V_(r) =the amplitude of the voltage detected by the loop element on theother side of the loop element having the largest detected voltage.

For example, if

P_(s) =7 units

L_(d) =2 units

V_(r) =4 volts

V_(s) =9 volts, and

V_(t) =5 volts

Then: ##EQU2##

The position of stylus 45 along both axes of measurement is obtained inthis manner.

As can be appreciated, the accuracy of the interpolation techniquedescribed above is based on the assumption that a curve of the amplitudeof the electromagnetic field along an axis of measurement is parabolicin shape in the region near the top of the curve. The assumption,however, is dependent somewhat on the exact geometry of the loop width,loop size, loop length, coil diameter and coil length etc. By suitablechoices of these parameters relative to each other, the curve of theelectromagnetic field can be made to be an almost perfect parabolicshape. An example of parameters that will produce a curve that isparabolic in shape at the top region is as follows:

Loop width 0.8"

Loop spacing 0.2"

Coil height from grid 0.5"

Coil length 0.6"

Coil diameter 0.3"

other combinations may be obtained by emperical measurements or bycomputer simulations.

The parabolic shape of the top region of the curve may be furtherimproved by using a coil that is frusto-conically shaped rather thancylindrically shaped.

In order to improve the performance of the loops at their tops or outerends (i.e. where the two longitudinal loop sections are connected to thetransverse loop sections) the shape of the outer ends may be slightlytapered rather than rectangular as shown. The bottoms of the loops (i.e.where they are coupled to the common conductor and multiplexor) may alsobe slightly tapered.

The plurality of loops making up each set need not all terminate on thesame side of the tablet. Some loops may terminate on one side and otherson the other side, it only being necessary that the loop areas andshapes be equal.

Instead of having a separate amplifier, demodulator and filtercombination and A/D converter for each set of loops defining one axis ofmeasurement, a single amplifier, demodulator and filter combination andA/D converter may be shared by both sets of loops through a suitablemultiplexing arrangement.

The embodiment of the present invention is intended to be merelyexemplary and those skilled in the art shall be able to make numerousvariations and modifications to it without departing from the spirit ofthe present invention. All such variations and modifications areintended to be within the scope of the present invention as defined inthe appended claims.

What is claimed is:
 1. A digitizing tablet system comprising:a. ahousing having a two dimensional work surface, b. a grid structure insaid housing, said grid structure including a first plurality of spacedparallel U-shaped conductive loop elements equal in size disposed in aplane parallel to said work surface and defining a first coordinate axisand a second plurality of spaced parallel U-shaped conductive loopelements equal in size disposed in a plane parallel to said work surfaceand defining a second coordinate axis, c. a magnetizable coil indicatinga position relative to said first and second coordinate axes of saidgrid structure, d. means for providing an alternating voltage signal toenergize said magnetizable coil so as to produce by electromagneticinduction voltage signals in the loop elements of said grid structurerelated to the position indicated by said coil relative to the axes ofsaid grid structure, and e. processing means for processing said inducedvoltage signals to determine the position indicated by said coilrelative to an axis of said grid structure as a function of threepositions defined by a U-shaped loop having the largest induced voltageand the U-shaped loops on either side thereof.
 2. The digitizing tabletsystem of claim 1 wherein said processing means comprises means fordemodulating said induced voltage signals and means for digitizing saiddemodulated signals.
 3. The digitizing tablet system of claim 1 whereinthe loop elements in each plurality of loops are arranged in anoverlapping relationship.
 4. The digitizing tablet system of claim 1wherein said magnetizable coil has a diameter which is less than thewidth of a loop element.
 5. The digitizing tablet of claim 4 whereinsaid processing means includes means for defining a parabolic curvecontaining the largest induced voltage measured from a U-shaped loop andthe induced voltages measured from the U-shaped loop on either sidethereof and means for determining the position relative to an axis ofthe peak of said parabolic curve.
 6. A digitizing tablet systemcomprising:a. a housing having a two dimensional work surface, b. a gridstructure in said housing, said grid structure including a firstplurality of spaced parallel U-shaped conductive loop elements equal insize disposed in a plane parallel to said work surface and defining afirst coordinate axis and a second plurality of spaced parallel U-shapedconductive loop elements equal in size disposed in a plane parallel tosaid work surface and defining a second coordinate axis, c. amagnetizable coil having a diameter less than the width of a loopelement, said coil indicating a position relative to said first andsecond coordinate axes of said grid structure, d. means for providing analternating voltage signal to energize said magnetizable coil so as toproduce by electromagnetic induction voltage signals in the loopelements of said grid structure related to the position indicated bysaid coil relative to the axes of said grid structure, e. means fordemodulating said induced voltage signals, f. means for digitizing saiddemodulated signals, and g. processing means for processing saiddigitized signals to produce data signals corresponding to the positionindicated by said coil relative to the axes of said grid structure. 7.The digitizing tablet system of claim 6 wherein said processing meansproduces data signals corresponding to the position indicated by saidcoil as a function of the positions of the U-shaped loop having thelargest induced voltage and the positions of the U-shaped loop on eitherside thereof.
 8. The digitizing tablet system of claim 7 wherein saidfunction defines a peak of a parabola including the U-shaped loop havingthe largest induced voltage and the U-shaped loop on either sidethereof.
 9. The digitizing tablet system of claim 6 wherein the loopelements in each plurality of loops are arranged in an overlappingrelationship.
 10. A digitizing tablet system comprising:a. housinghaving a two dimensional work surface, b. a grid structure in saidhousing, said grid structure including a first plurality of overlappingparallel U-shaped conductive loop elements equal in size disposed in aplane parallel to said work surface and defining a first coordinate axisand a second plurality of overlapping parallel U-shaped conductive loopelements equal in size disposed in a plane parallel to said work surfaceand defining a second coordinate axis, c. a magnetizable coil having adiameter less than the width of a loop element, said coil indicating aposition relative to said first and second coordinate axes of said gridstructure, d. means for providing an alternating voltage signal toenergize said magnetizable coil so as to produce by electromagneticinduction voltage signals in the loop elements of said grid structurerelated to the position indicated by said coil relative to the axes ofsaid structure. e. means for demodulating said induced voltage signals,f. means for digitizing said demodulated signals, and g. processingmeans for processing said digitized signals to determine the positionindicated by said coil relative to an axis of said grid structure as afunction of three positions defined by a U-shaped loop having thelargest induced voltage and the U-shaped loops on either side thereof.11. The digitizing tablet system of claim 10 wherein said function isdefined by a peak of a parabola defined by said three positions.